Syntactic Sugar for Virtual Constructors?

[Janice Caron]

On 28/02/2008, BCS <BCS at pathlink.com> wrote:
>         AbstractBaseClass this(string s, int i)
>         {
>                 switch(s)
>                 {
>                         case "Foo": return new DerivedClass(i);
>                         ...
>                         default: return null;
>                                 // check code causes exception
>                 }
>         }

The switch is the one thing that you /must not have/. The factory
cannot possibly know all possible (present and future) derived
classes. Putting that switch statement there prevents anyone without
access to the source code from making new derived classes, making the
mechanism completely unworkable for a library. Also, this mechanism
needs to import all of the module(s) where the derived class(es)
is/are defined, which is also bad. Also, the function above would
cause "executable bloat", whereby every possible derived class will
end up in every executable which calls that function, whether needed
or not.

The good way to implement a factory function is to call the function
object.Factory(). That way, the factory function needs /no knowledge/
of the derived class, beyond it's class name, extendability is
retained, and executable bloat is avoided.